Conventionally, four-wheel drive automobiles have front and rear drive axles which are driven by a common engine. Between the front and rear drive axles, there is provided an inter-axle differential gear mechanism which makes it possible to distribute the engine driving power transmitted from a transmission to the front and rear axles so as to absorb any difference in rotating speed between the front and rear axles which may be produced when the automobile passes through a curved path. In this type of automobile, it is desirable to provide a locking device for the inter-axle differential gear mechanism because the automobile may otherwise encounter difficulties in getting out of a loose soil once either of the front or rear wheels get into such loose soil because the engine driving efforts may totally be transmitted to the wheels which are in the loose soil thereby causing racing of the wheels.
Therefore, it has been proposed for example by the U.S. Pat. No. 3,400,777 to provide a four-wheel drive motor vehicle transmission unit including an inter-axle differential gear unit provided with a differential gear lock mechanism for locking the inter-axle differential gear. In the transmission unit as proposed by the U.S. Patent, the transmission output shaft is connected through a pair of bevel gears with a tubular shaft which is perpendicular to the transmission output shaft. The tubular shaft is connected at one end with a pinion carrier of a planetary type inter-axle differential gear mechanism, the sun gear of the differential gear mechanism being connected with a second tubular shaft passing through the first tubular shaft and the ring gear with the input member of a rear differential gear mechanism having a pair of axially aligned rear axles. One of the rear output shafts is passed through the second tubular shaft so as to be connected with one rear wheel hub. The other end of the first tubular shaft is provided with a one-way overrunning clutch device for locking the first tubular shaft to the second tubular shaft when the differential action tends to vary by more than a predetermined amount to thereby lock the inter-axle differential gear mechanism. The first tubular shaft is further connected through a pair of bevel gears with a propeller shaft which is adapted to be connected with a front differential gear mechanism for front axles.
It should however be noted that this type of arrangement is disadvantageous in that it makes the length of the transmission unit long due to the structure of applying the driving power to the pinion carrier of the inter-axle differential gear mechanism. More specifically, when the pinion carrier is used as an input member, it must be connected at a side thereof with a driving member so that any interference is avoided with the ring gear which is located radially outside the pinion carrier and the sun gear which is located radially inside the pinion carrier. Thus, there must be provided a certain axial space between the inter-axle differential gear mechanism and the driving member. Besides, in the above arrangement, since the rear differential gear mechanism is located at one end of the transversely extending tubular shaft, there must be substantial difference in length between the rear axles. Further, there is a limit in positioning the propellar shaft with respect to the transmission unit, since the bevel gear for driving the propellar shaft must be located on the first tubular shaft axially outwards of the one-way overrunning clutch. The causes restrictions in laying out the propellar shaft.